Huge amount of work. Does not even nearly compile, but it's nearer.

src/c/payloads/stack.h

@@ -0,0 +1,47 @@
+/**
+ *  payloads/stack.h
+ *
+ *  a Lisp stack frame.
+ *
+ *  Sits in a pso4.
+ *
+ *  (c) 2026 Simon Brooke <simon@journeyman.cc>
+ *  Licensed under GPL version 2.0, or, at your option, any later version.
+ */
+
+#ifndef __psse_payloads_stack_h
+#define __psse_payloads_stack_h
+
+#include "memory/pointer.h"
+
+/*
+ * number of arguments stored in a stack frame
+ */
+#define args_in_frame 8
+
+/** 
+ * @brief The maximum depth of stack before we throw an exception.
+ *
+ * `0` is interpeted as `unlimited`.
+ */
+extern uint32_t stack_limit;
+
+/**
+ * A stack frame. 
+ */
+struct stack_frame_payload {
+    /** the previous frame. */
+    struct pso_pointer previous;
+    /** first 8 arument bindings. */
+    struct pso_pointer arg[args_in_frame];
+    /** list of any further argument bindings. */
+    struct pso_pointer more;
+    /** the function to be called. */
+    struct pso_pointer function;
+    /** the number of arguments provided. */
+    uint32_t args;
+    /** the depth of the stack below this frame */
+    uint32_t depth;
+};
+
+#endif